1. Field of the Invention
The present invention relates to a technique to improve image quality in the case where, for example, a frame rate is converted twice in such a manner that a frame frequency is converted from 50 fps (frames per second) to 100 fps in a TV (television) receiver. In particular, the present invention relates to an image signal processing unit and a method of processing an image signal, which improve the image quality in the case where, for example, an image in which a normal signal (for example, a telop) with a frame rate of 50 fps is superimposed on a film signal with a frame rate converted from 25 fps to 50 fps is input.
2. Description of the Related Art
In the TV receiver, for example, when an image with a frame rate of 50 fps such as a PAL (phase alternating line) signal is displayed, there is a shortcoming that a whole screen flickers, that is, “plane-flicker disturbance” occurs in, for example, a CRT-based display apparatus. To solve this shortcoming, a frame rate conversion which solves the plane-flicker disturbance by setting the frame frequency high has been used. In this frame rate conversion, for example, the frame frequency is converted from 50 fps to 100 fps. Such a frame rate conversion is also used to improve blurring of a moving image in, for example, a display apparatus of a liquid crystal method.
On the other hand, for example, an image signal used for a movie shoot has the frame frequency lower than that of a typical TV signal, and is, for example, a film image signal with a frame rate of 25 fps. For this reason, a process called a television-cinema conversion (telecine conversion) in which a signal format of a film image signal is converted to that of the typical TV signal has been performed. As an example of the telecine conversion, for example, there is a 2-2 pull-down in which the film image signal with a frame rate of 25 fps is converted to the image signal of the PAL method with a frame rate of 50 fps.
FIG. 5 indicates an example of a film image 10 configured by converting the frame rate from 25 fps to 50 fps through a 2-2 pull down. FIG. 5 is an example of the film image 10 in which an object moves in a right direction in a screen. Although the TV signal has a frame rate of 50 fps, the image content is the a film image 10 configured by converting the frame rate from 25 fps to 50 fps through a 2-2 pull down. Thus, each two successive frames in the TV signal have the same image content. Accordingly, in the case where the object moves in the screen, as indicated in FIG. 5, a first frame F1 and a second frame F2 have the same image content, and the object is in the same position. The position of the object moves in a third frame F3. The third frame F3 and a fourth frame F4 have the same image content, and the object is in the same position in the third frame F3 and the fourth frame F4. The position of the object moves in a next fifth frame F5. In this manner, in the film image, since the image moves with a frame rate of 25 fps in the screen, there is a shortcoming that the motion of the image is not visually smooth. This unsmooth motion is called “judder”.
To solve this shortcoming, “a motion correction process” is added. In the motion correction process, a motion vector of an input image is obtained, and an image which is shifted in accordance with the amount of the obtained motion vector is inserted as an interpolation frame image, thereby realizing smooth motion. This process has been widely used.
Here, with reference to FIGS. 6A and 6B, operation to smooth the motion of the image will be described. In this case, the input image with a frame rate of 50 fps is the film image configured by converting the frame rate from 25 fps to 50 fps through a 2-2 pull down, and the frame rate is converted from 50 fps to 100 fps by adding the motion correction process. FIG. 6A indicates the input image before the frame rate conversion is performed, and FIG. 6B indicates the output image after the frame rate conversion involving the motion correction process is performed. In FIGS. 6A and 6B, the vertical axis indicates time, and the horizontal axis indicates a position of the image in the horizontal direction.
The content of the input image indicated in FIG. 6A is substantially the same as that of the film image 10 indicated in FIG. 5. Similarly to FIG. 5, in the image content, the object moves in the right direction. This input image is configured by converting the frame rate of the film image 10 from 25 fps to 50 fps through a 2-2 pull down. In the input image, each two successive frames have the same image content. Thus, when the object moves in the screen, as indicated in FIG. 6A, the image of the first frame F1 and the image of the second frame F2 are in the same position, and the object largely moves in the third frame F3. The fourth frame F4 is displayed in the same position as that of the third frame F3. At this time, between the first frame F1 and the third frame F3 which is two frames after the first frame F1 in the input image, the motion vector of the image is obtained by, for example, block matching method as a known method. The motion vector is obtained for each pixel unit, or for each block having a certain size. The obtained motion vector amount is regarded as A.
As indicated in FIG. 6B, in accordance with the obtained motion vector amount A, interpolation frame images are formed by shifting the image of the first frame F1 in the input image by the amounts of A×¼, A× 2/4, and A×¾. These interpolation frame images obtained by shifting are displayed as a frame F1′, a frame F2, and a frame F2′, respectively, in the output image with a frame rate of 100 fps, and thereby smooth motion with a frame rate of 100 fps is realized.
FIG. 7 indicates an example of the configuration of an image signal processing unit of the related art which performs the frame rate conversion as descried above. In the image signal processing unit, an input signal 152 is input from an input terminal 151 to a frame rate conversion circuit 153. As indicated in FIGS. 5 and 6A, the input signal 152 is a film image signal configured by converting the frame rate from 25 fps to 50 fps through a 2-2 pull down (as an image format, an image signal with a frame rate of 50 fps). The frame rate conversion circuit 153 includes a first frame memory 154, a second frame memory 155, and a third frame memory 156. The frame memories 154, 155, and 156 are image memories each temporarily storing image data of one frame. The frame rate conversion circuit 153 also includes a film phase detection circuit 160, a motion vector detection circuit 161, a switch 162, and a double rate conversion/image shift circuit 166.
In the frame rate conversion circuit 153, the image is delayed with the first frame memory 154 and the second frame memory 155 which are frame memories for two frames, and, by using the input signal 152 and a two-frame delayed signal 158, a motion vector 165 between two frames is obtained with the motion vector detection circuit 161 by block matching method or the like. Moreover, by using the input signal 152 and a one-frame delayed signal 157, a correction timing in accordance with a film phase of the input image is obtained with the film phase detection circuit 160. The above-mentioned two-frame delayed signal 158 is further delayed by one frame with the third frame memory 156, thereby obtaining a three-frame delayed signal 159. The two-frame delayed signal 158 and the three-frame delayed signal 159 are supplied to the switch 162. In the switch 162, by inverting the switching operation for every one frame, the image of the odd-numbered frame in the input image is generally selected, and regarded as a selection signal 163.
On the other hand, the obtained motion vector 165 and a correction timing signal 164 are input to the double rate conversion/image shift circuit 166. By the operation of the switch 162, the selection signal 163 in which the image of the odd-numbered frame in the input image is generally selected is input to the double rate conversion/image shift circuit 166. In the double rate conversion/image shift circuit 166, the frame rate of the image of the odd-numbered frame is converted so that the converted frame rate is twice that of the input signal, and then an image position is appropriately shifted in response to the motion vector 165 and the correction timing signal 164. Thereby, the output signal 167 whose motion is improved to be smooth is obtained.
As described above, the image is appropriately moved when the motion vector is obtained and the frame rate is converted, and thereby the smooth motion is realized. Japanese Patent Publication Nos. 3596521 and 3855761 each discloses the invention regarding the image signal processing unit which performs such a frame rate conversion.